A plug-in unit for an electronic ignition system in a motor vehicle is described in the publication Bosch-Zunder, No. 3/1977, page 6. After installation, the function and operation of the plug-in unit is fully defined and it is not possible to make any changes in its circuitry without great technical difficulty and attendant cost. On some occasions trouble has resulted from a spark produced just when the engine is stopped by opening the ignition switch.